bun:test: give each VM its own fake clock - #38740

The old `AtomicU64` encoding used NaN as the "not mocked" sentinel, so `set_wall_ms(NaN)` left `wall_ms()` returning `None`; the new `Cell<Option<f64>>` stores `Some(NaN)`, so `useFakeTimers({now: NaN})` / `{now: new Date('invalid')}` now makes `milli_timestamp_allow_mocked_time()` return NaN (feeding `Bun.cron`) while `JSMock__setOverridenDateNow(NaN)` still means "no override" — the two sides now disagree on whether the wall clock is mocked. Consider either short-circuiting NaN → `None` in `se

nit: a `setInterval` that is *currently firing* when its own callback calls `useFakeTimers()` survives this drop — `clear()` only walks the heap, and the interval was already popped by `execute_until`, so `run_impl` still sees `state == FIRED` after the callback and reschedules it into the new heap at the pre-callback `time_before_call`. The reinstall-inside-callback test uses `setTimeout` so it doesn't hit this. Very much an edge case, but the doc comment slightly overstates what is dropped.